(a) Field of the invention:
The present invention relates to a playback optical system for video discs.
(b) Description of the prior art:
Since a video disc optical system is a diffraction-limited optical system, aberrations in a read-out lens for video discs to be used for reading information must be corrected as favorably as those in an objective lens system for microscopes. In addition, light weight is another requirement for a read-out lens for video discs and, therefore, it must consist of lens components in a number as small as possible. Accordingly, it is very hard to design a favorable read-out lens for video discs.
On video discs, information is recorded in rows of phase grooves called pits. In order to increase information density, it is necessary to minimize intervals of the pits and tracks (distance between neighboring rows of pits). The pit width is narrower than diameter of the beam spot used for reading information from the discs, and lower limits (minimum values) of the pit interval and tracks interval are determined by diameter of the spot. In order to increase information density, it is therefore necessary to minimize diameter of the beam spot to be used for reading out information. In order to minimize diameter of the beam spot, it is generally practiced to use a lens system having a large numerical aperture (N.A.) as the read-out lens for video discs. When a large N. A. is selected for an objective lens, working distance is shortened and it becomes rather hard to correct aberrations favorably. If it is attempted to solve these problems by increasing number of lens element composing the read-out lens, it will increase weight of the lens and result in other disadvantages for a playback lens system for video discs.
Moreover, pit width must be within a definite range in order to read information with the ordinary type of the conventional read-out lens. For this reason, a writing lens used for recording information must also have a large N. A. and favorably corrected aberrations.